Direct plasmadynamic conversion of plasma thermal power to electricity

Abstract

The generation of electrical energy using direct plasmadynamic conversion (PDC) is studied experimentally for small-scale, chemically-assisted plasmas (CA-plasma) for the first time. Glow discharge and microwave-generated plasma sources are operated at power levels on the order of a few to 50 W in the discharge case and up to 12.83 W/cm/sup 3/ in the microwave case. Extracted power approaching 1/4 W has been achieved as a demonstration. It is envisioned that such a system may be readily scaled to a few hundred Watts to several tens of kilowatts output power for microdistributed commercial applications (e.g., household, automotive, light industry, and space based power). Three-quarter inch long by 0.040-in diameter cylindrical PDC electrodes have been tested in a 10-50 W direct current, glow discharge plasma device with He or Ar as the working gas at 0.3-3.0 torr. The PDC anode was magnetized in the range of 0-700 G with a 1.5-in water cooled Helmholtz electromagnet. Open circuit voltages up to 6.5 V were obtained across the PDC electrodes at 1 torr He and 350-G field. The collector voltage was shown to be a function of applied magnetic field strength B and peaking at about 300 G. A variety of resistive loads were connected across the PDC electrodes, extracting continuous electrical power up to 0.44 mW. The power/load curve peaks at 0.44 mW for a 20 k/spl Omega/ load indicating the impedance matching condition with the plasma source. The most severe limitation to collector output performance is shown to be plasma conductivity. Collector power drops sharply with increasing neutral gas fill pressure in the glow discharge chamber at constant discharge current indicating that electron collisions with neutral gas atoms are responsible for the reduction in conductivity. Scale-up to higher power has been achieved with the use of a microwave plasma generator. A 0.75-in long by 0.094-in diameter PDC anode was magnetized to /spl sim/140 G resulting in open circuit PDC voltages in excess of 11.5 V for He plasmas at /spl sim/0.75-1 torr and 50 sccm flow. Due to higher conductivity, load matching was now obtained at /spl sim/600 /spl Omega/. Langmuir probe results indicate good agreement between the conductivity change and the electron to neutral density ratio scale-up. For this source and electrode configuration, PDC power as high as /spl sim/200 mW was demonstrated in He at 0.75 torr for a microwave input power density of /spl sim/8.55 W/cm/sup 3/. Considering an electron mean-free path as the scale for collector probe influence in the plasma, the peak extracted power density is /spl sim/1.61 W/cm/sup 3/, corresponding to a volumetric conversion efficiency of /spl sim/18.8%.